1. Field of the Invention
This invention relates to surge protection for components in a terminal, such as a telephone set. Protection is required against AC surges which can arise due to downed AC power lines falling across telephone lines, and other occurrences.
2. Related Art
Originally, such protectors comprised a wire wound resistor. However, an overload could cause the resistor to break down in a manner which is a fire hazard. Wire wound resistors were replaced by a thick film resistor printed onto a surface of a ceramic substrate. The substrate is at least 0.1 inches thick and the resistor is a continuous layer. To trim the resistor, the layer of resist material is abraded locally, the abrasion extending through the layer to the substrate.
Developments in materials and processes enable the substrate thickness to be reduced to 0.06 inches, but disadvantages such as 100% testing resulted.
The thickness of the substrate reflects the ability of the substrate to withstand the heating resulting from a surge. Normally, a thinner substrate will disintigrate at a lower surge voltage than a thicker substrate. The effect is amplified by variation in the distribution of the surge effect over the substrate area, with an associated non-symmetrical heating of the substrate. Copending application Ser. No. 899,403, assigned to the same assignee, discloses a surge protector employing a serpentine pattern.